Device for stacking sheets, in particular sheet-fed paper or cardboard sheets delivered onto pallets

ABSTRACT

Devices for stacking sheet-fed paper or cardboard sheets ( 1 ) on pallets ( 4 ) are known to have a vertically stationary conveyor ( 2 ) at a height above the maximum stack level above the floor and a storage platform ( 5 ) which can be raised and lowered upon which the stacks are formed. An optical sensor ( 17 ) which senses the stack level is arranged at the input side prior to the stacking area to control the lowering of the storage platform ( 5 ). According to the invention, at least three optical sensors ( 17 ) are disposed below the conveyor level at a slight distance from each other in a vertical line. The maximum vertical distance between the two sensors ( 17 ) is 20 mm. By activating any two sensors ( 17 ), the drop in height of the sheets ( 1 ) can be set depending on the type of sheet and without the need for extensive adjustment upon a change in the type of sheet.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage of PCT/EP98/01342 filed Mar. 7,1998 and based upon German national application 197 11 406.7 filed Mar.19, 1997 under the International Convention.

TECHNICAL FIELD

The invention relates to a device for stacking sheets onto pallets,particularly paper or cardboard sheets fed in a scale-like (overlapping)manner.

BACKGROUND OF THE INVENTION

For the stacking of sheets downstream of a crosscutter it is known touse devices which have a raisable and lowerable storage platform, onwhich pallets for receiving the stacks have been laid. The sheets areconveyed to the stacking area and laid on the upper side of the stack bya vertically stationary feeder, arranged above the ground at a heightwhich is greater than the maximal stack level. During stacking thestorage platform is continuously lowered corresponding to the growth ofthe stack, so that the drop height of the sheets from the feeder to theupper side of the stack remains constant, insuring a trouble-freestacking with precisely aligned edges.

For a stack replacement the storage platform with the full pallet islowered to the ground level. After the full pallet is removed andreplaced with an empty pallet, the platform is raised to a levelslightly below the feeding level. In order not to interrupt the feedingof sheets during a stack replacement, devices with an auxiliary stackingplatform are known, which support the forming stack while the finishedstack is removed. The empty pallets are then moved to a level directlybelow the auxiliary stacking platform and take over the partial stackformed on the auxiliary stacking platform.

STATE OF THE ART

From DE 38 23 806 C2 a stacking device is known, wherein the control ofthe lowering motion of the storage platform is performed by means of anoptical sensor, which is arranged on the incoming side of the stackingarea and detects the stack height.

The document EP-A-0 211 996 discloses a generic stacking device whereinlaterally next to the storage platform three optical sensors arearranged vertically one above the other, which are all active duringoperation. The individual sensors are arranged at distances of 10-20 mm,20-40 mm and 40 to 60 mm underneath the upper end of the feedingconveyor and serve for controlling the lowering speed of the storageplatform.

From the EP-A-0 149 737 a stacking device is known, wherein at thebeginning of the stacking area laterally five initiators are arrangedvertically one above the other at a height at which the storage platformis supposed to be when a stack replacement is to take place. Each of theinitiators can be separately activated and can trigger a braking devicefor stopping the supplied flow of sheets, in case the storage platformhas reached this position due to the curvature of the sheets, althoughthe preset number of sheets to be deposited has not been reached.

In practice it has been proven that the drop height of the sheets, whichhas to be kept substantially constant, has to be set differently,depending on the type of paper or cardboard. Therefore in case of apaper-type change, the optical sensor registering the desired stacklevel has to be reset in another vertical position in a complicatedmanner.

OBJECT OF THE INVENTION

It is the object of the invention to improve a generic stacking device,so that the drop height of the sheets can be set at various valueswithout effort, in a simple manner.

SUMMARY OF THE INVENTION

According to the invention, at least three optical sensors for the upperstack side are arranged vertically one above the other and areseparately actuatable. Each time two selected sensors establish themaximal drop height deviation and in this way make possible a control ofthe storage platform in such a manner that an upper maximal value cannotbe exceeded and so that it can not fall below a lower minimal value.

In accordance with the invention, a device for stacking sheets ontopallets, particularly paper or cardboard sheets fed in an overlappingmanner comprises a vertically stationary feeding conveyor for the sheetsarranged above the floor at a height exceeding the maximum stackinglevel, a raisable and lowerable storage platform on which the stacks areformed, and at least three optical sensors arranged vertically one abovethe other at a slight distance below the feeding level. The opticalsensors can be separately activated and are arranged on the incomingside upstream of the stacking area, whereby the vertical distancebetween two sensors amounts to a maximum 20 mm. More particularly, thevertical distance between two sensors can range between 5 mm and 10 mm.The sensors can be reflecting optical sensors.

BRIEF DESCRIPTION OF THE DRAWING

The drawing serves for the clarification of the invention with the aidof an embodiment example shown in a simplified manner.

FIG. 1 is a side view of the basic construction of a stacking deviceaccording to the invention;

FIG. 2 is an enlarged side view of the arrangement of the opticalsensors in relation to the stack; and

FIG. 3 is a perspective view in which shows the arrangement of theoptical sensors in the discharge section in a view inclined with respectto the travel direction of the sheets.

WAYS FOR IMPLEMENTING THE INVENTION

The subsequently described embodiment serves for stacking of papersheets 1 continuously fed in a scale-like manner arranged downstream ofa crosscutter. The sheets are fed to the stacking area by a feedingconveyor 2 designed as a belt conveyor, which is arranged above theground at a height exceeding the maximal stacking level. The stacks 3are formed on pallets 4, which are laid upon a storage platform 5. Thestorage platform 5 is suspended in the frame 6 by traction chains 8. Bymeans of the traction chains 8 the platform can be raised from floorlevel up to the area of the feeding level 7 of the sheets 1 and can belowered. Furthermore the stacking device comprises the known elementsfor insuring that the sheets 1 are deposited on the stack 3 withcorrectly aligned edges: A stop plate 9 for the advancing or leadingsheet edge and lateral vibrating plates 10 which act on the longitudinalsheet edges.

Between the end of the belt conveyor 2 marked by the runoff side of theguide roller 11 and the stacking area, at the height of the feedinglevel, the so-called discharge section is arranged, whose elementsinsure a trouble-free transfer of the sheets 1 to the stack 3. The areaof the discharge section is shown on a larger scale in FIG. 3. Followingthe belt conveyor 2 is a rigid plate 12 extending vertically downwardlyfrom the feeding level, which defines the stacking area on its incomingor upstream side. The trailing sheet edges are aligned on the plate 12.At the height of the feeding level 7, the plate 12 has recesses whereinthe discharge rollers 13 are supported, which together with the blowingair nozzles 14 insure a safe transport of the sheets 1 into the stackingarea. Between the discharge rollers 13 there are further recesses 15,one of which serves for lodging the device for detecting the stacklevel, which will be subsequently described in greater detail.

The device for detecting the stack level consists of a box-like housing16, in whose frontal side facing the stacking area at least 3 opticalsensors, five optical sensors in the example shown, are arrangedvertically one above the other. Preferably reflecting optical sensorsare used for the optical sensors 17, wherein light senders and lightreceivers are located in a housing (housing 15). The vertical distancebetween two sensors 17 is a maximum of 20 mm, preferably between 5 mmand 10 mm, in the present example 7.5 mm. The uppermost sensor 17 islocated at a slight distance below the feeding level, whereby thedistance is sufficiently big to allow for the upwards motion of theplatform 5 to stop in time, so that the stack upper side remains belowthe feeding level. Therefore the vertical position of the uppermostsensor 17 is selected so that the stack upper side can be controlled sothat it does not fall below the desired minimal distance from thefeeding level, independently of the sheet material to be stacked. In theembodiment shown example the uppermost sensor 17 is located at 10 mmbelow the feeding level 7.

As shown in FIG. 3, the sensors 17 are staggered in two vertical lines,in order to insure the desired slight vertical distance of two sensors17 from each other. The signal conductors 18 are in the housing 15 andare connected to a control unit not shown in the drawing for the controlof the vertical motion of the storage platform 5.

During the operation of the stacking device two of the five sensors 17are activated, the rest remaining inactive. The uppermost of the twoactive sensors 17 determines the minimal distance of the stack upperside from the feeding level 7, the lower active sensor 17 determines themaximal distance. The vertical position of the stack upper side iscontrolled through a vertical motion of the storage platform 5, so thatbasically it is kept within the desired vertical range between the twoactive sensors 17. The control takes place suitably by means of a twopoint point adjustment system, which when the upper position is upwardlysurpassed triggers a lowering motion of the storage platform 5, and whenit falls below the lower position, triggers a lifting motion thereof.

If due to a type change of the sheets 1 to be stacked another dropheight and/or another range for the acceptable drop height differencehas to be set, the two corresponding sensors 17 are activated. In thesimplest form with three sensors 17, the minimal drop height, as well asthe adjustment range of the drop height can be set at two differentvalues. When five sensors are used, as is the case in the embodimentexample, multiple possibilities are offered to adjust the drop height byactivating the two sensors 17 within a type-depending range, wherein atrouble-fee deposition of the sheets 1 is achieved. Therefore acomplicated readjustment of the sensors 17 detecting the stack level incase of a type change is not required. It is merely necessary toactivate the two suitable sensors 17.

We claim:
 1. A device for stacking sheets on pallets, comprising: avertically stationary feeding conveyor for the sheets arranged above afloor at a height exceeding a maximum stacking level; a raisable and alowerable storage platform adapted to receive a pallet on which a stackis to be formed located at a downstream end of said conveyor; and atleast three optical sensors arranged vertically one above the other at aslight distance below a feeding level at said downstream end of saidconveyor and upstream of said platform at an inlet side thereof fordetecting a height of the stack formed on said platform, the opticalsensors being separately activatable so that selectively two of saidsensors are activated for a particular sheet type while all others ofsaid sensors are inactive, the vertical distance between two sensorsbeing a maximum of 20 mm.
 2. The device according to claim 1 wherein thevertical distance between two sensors ranges between 5 mm and 10 mm. 3.The device according to claim 1 wherein the sensors are (17) reflectingoptical sensors.